Tissue factor (TF) is the high affinity cell surface receptor and obligatory cofactor for plasma coagulation factor VII. Purification of TF from human brain and placenta has recently been reported to yield both a monomeric 47 kDa protein as well as a 58 kDa heterodimeric form. In this study, the TF glycoprotein was isolated from human brain by immunoaffinity chromatography using a newly developed monoclonal antibody, TF8-5G9, and was compared to TF isolated by factor VII-affinity chromatography. Except for the greater efficiency of the immunoaffinity method, both methods yielded TF with similar specific activities, and both preparations contained the monomeric and heterodimeric forms of TF. The 58 kDa form was established to be a disulfide-linked heterodimer composed of TF and the alpha chain of hemoglobin. From these results and from studies of immunoprecipitation of TF from cultured fibroblast cells, we conclude that the 47 kDa monomeric form of TF is the naturally occurring cellular form of TF, and that heterodimer formation is a secondary event. The potential significance of the proclivity of TF to form a heterodimer with hemoglobin is discussed.